The objective of the proposed research is to discern the underling mechanisms by which exposure to certain non-genotoxic compounds of environmental concern can alter the expression of genes which are involved in normal cell proliferation and differentiation. The research project is designed to test the hypothesis that decreased DNA 5-methylcytosine content (i.e., hypomethylation of DNA) is a component of the mechanism involved in tumor promotion. Hypomethylation of DNA would be expected to increase the potential for gene expression. An increased potential for expression of genes involved in normal growth and development (i.e., proto-oncogenes) might be an underlying mechanism by which their aberrant expression is brought about in the multistep process by which a normal cell is transformed into a cancer cell. Alteration in DNA methylation represents a heritable epigenetic event which may be brought about by threshold- mechanisms. Mouse liver will be the experimental system, using the hybrid B6C3F1 mouse and its paternal C3H/He plus maternal C57BL/6 strains. The B6C3F1 and C3H/He strains are hepatoma-prone while the C57BL/6 is relatively resistant. The specific aims are primarily directed towards an assessment of the potential mechanisms by which non-genotoxic compounds, which appear to act as promoters of carcinogenesis, may alter the methylation status of the proto-oncogenes of interest, and secondarily, actual measurements of expression of these genes, e.g., levels of the respective mRNA's and protein products. Emphasis shall be placed on dose- response relationships as we assess the ability of non-genotoxic compounds to affect the capacity and fidelity of DNA maintenance methylation. The chemicals of interest include trichloroethylene (a non-genotoxic volatile organic compound of environmental concern because it is frequently detected in groundwater), and chlordane (a non-genotoxic pesticide). A unique aspect of this proposed research project is that it offers the potential to discern molecular mechanisms involved in chemical-induced aberrant gene expression while, at the same time, providing the type of information which is required in order to take a more rational approach towards evaluation of bioassay data (the B6C3F1 mouse is the strain required in the National Toxicology Program's carcinogen bioassay protocol). The need to incorporate knowledge regarding mechanism of action into the risk assessment process comprises an important aspect of environmental toxicology.